1. Field of the Invention
This invention relates generally to semiconductor devices, and more particularly, to apparatus for providing heat dissipation for such devices.
2. Discussion of the Related Art
FIG. 1 shows a typical prior art semiconductor structure 10 which will be described as background to the present invention. The semiconductor structure 10 includes a semiconductor body 12 in the form of a silicon substrate including a plurality of active devices such as transistors 14, each in turn including a source and drain and a gate. A plurality of conductive metal layers 16 in the form of a metal stack 17 are housed in a dielectric material 18 (such as silicon dioxide) on the semiconductor body 12, and the metal layers 16 are operatively connected to the active devices 14 and to each other by means of vias 20, all as is well-known. The metal layers 16 are also connected to conductive elements 22 in the form of solder bumps, by means of vias 24. The conductive elements 22 connect to a substrate 26, for example, a printed circuit board.
The operation of the active devices 14 causes heat generation and buildup in the semiconductor body 12. To dissipate heat from the semiconductor body 12, thermal grease (or thermally conducive elastomer) 28 is applied to the body 12 on the side thereof opposite the metal stack 17, and a heat sink 30 is mounted on the thermal grease 28. Heat in the semiconductor body 12 is transferred through the thermal grease 28 to the heat sink 30 and is removed from the sink 30 by operation of a fan 32, thereby avoiding heat buildup in the semiconductor body 12.
Modem semiconductor devices of this type include a number of metal layers, currently for example as many as seven, with this number expected to increase in the future. Heat buildup caused by current in a metal stack 17 with many layers has become a problem. Ironically, the problem is exacerbated by the industry movement to the use of copper metal layers, rather than aluminum, which permits a several fold increase in current density (to improve electrical performance and device density) without increasing electromigration problems, yet resistance of such copper conductors is only about xc2xd that of aluminum, so that heat generated in the stack 17 is significantly increased. The Joule heating effect is particularly great, as it varies as the square of the current, but is linear with resistance. Also, low dielectric constant materials in which the stack is housed are now supplanting silicon dioxide, and these low dielectric constant materials have considerably less thermal conductivity than silicon dioxide, which is already poor in this regard Thus, heat generated in the metal stack tends to be held in the region thereof, rather than dissipated therefrom.
While the heat sink 30 and fan 32 described above are effective in removing heat from the semiconductor body 12, such heat sink 30 is physically too far away (in the sense of thermal resistance) from the metal stack 17 to effectively dissipate heat from the stack 17. As a result, heat generated in the metal stack 17 is dissipated to a limited extent through the conducting elements 22 into the substrate 26, which has proven to be an inefficient heat dissipation system
Therefore, what is needed is a heat dissipation system which is effective for dissipating heat from the metal stack of a semiconductor structure, which system is also simple in design and manufacture.
The present invention is a semiconductor apparatus including a semiconductor body in the form of a silicon substrate having a plurality of active devices. A metal stack including a plurality of metal layers is operatively associated with the active devices. Portions of at least some of the metal layers are connected by vias which are in turn connected to a conductive elements in the form of a solder bumps, in turn connected to a substrate in the form of a printed circuit board Heat generated in the metal stack is dissipated through the vias, through the conductive elements, and to a heat absorbing member within the substrate, and to an additional heat sink outside the substrate, connected to the heat absorbing member within the substrate by another via. The vias are positioned at regular intervals relative to the metal stack so as to promote effective heat removal from the metal stack.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this Invention simply by way of the illustration of the best mode to carry out the invention As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.